Gripping mechanisms are generally designated to grip a workpiece at a first place, to move it to a second place and to set it down there. The gripping mechanisms have for this purpose gripping means which clamp the workpiece between themselves. Among the movements which are to be carried out in practice are also pivoting movements about a horizontal axis, or about inclined axes with a limited horizontal projection. Such movements change the orientation of the gripping means with respect to the direction of the acceleration due to gravity, and this results in the weight of the gripping means contributing in a position-dependent manner to the clamping force and/or in changing the distance of the gripping means from the pivot axis. A change in the clamping force is undesired during transport of breakable articles or also during transport of easily deformable articles. The latter case exists for example in spot-welding tongs which are used at the same time for transport purposes, in which the part of the workpiece lying between the electrodes, which at the same time act as the gripping means, is plasticized by the welding operation. In order to prevent an uncontrolled deformation of the plasticized material volume during the pivoting of the welding tongs, due to the changing relatively large weight component of the gripping means (and the parts of the drives rigidly moved along with the gripping means), with conventional welding tongs pivoting of the workpiece must be delayed until the welding site has substantially cooled off. This means a considerable reduction in the productive capability of the welding mechanism.
The present invention is intended to further develop a pivotal gripping mechanism of the above-mentioned type so as to compensate for the change of the weight component of the moved parts of the gripping mechanism, which change occurs during the pivoting.